Lambda function, arguments and logic in c++ [duplicate]

Question

I am new to using lambda functions in C++. I have been researching the web, and found several articles, explaining the syntax and purpose of lambda function, but I have not come found articles which are clearly giving an explaining how to write the inner logic of a lambda function.

For example

During sorting a vector in c++ in decreasing order:

sort(v1.begin(), v1.end(), [](const int &a, const int &b){return b < a;});

I write the above code. Here, I have a few questions:

Why do I only provide two parameters in the lambda function? Why not three? or why not I give all the n parameter(n is size of vector) and do a logic? I am not trying to find maximum of two elements, I am trying to sort a vector, why should I only consider two values?
Why does a > b gives descending order? Why not b > a? Are there any kind of ordering inside the lambda function?
The return value in the above lambda function is either false(0) or true(1)? Why do I only have to return false(0) or true(1) to sort? Why can't I return a character to sort, like let's suppose for return value 'a' it is ascending and return value 'd' it is descending?

Again

During finding the max even element

itr = max_element(v1.begin(), v1.end(), [](const int &a, const int &b){
            if (isEven(a) && isEven(b)) {
                return (a < b);
            } else
                return false;
        }
        );

I am returning b > a. Rather than a greater than b. ???

Any suggestion would be greatly appreciated.

Sounds like you could use a good C++ book. A lot of your questions, and many more, are covered in them. — NathanOliver
– NathanOliver, Commented Sep 23, 2021 at 14:55
Most of these questions aren't really about the lambda specifically, they're about the third argument to std::sort. Specifically, the third overload here. sort(begin, end, comp) is saying "sort the range [begin, end) from smallest to largest, using comp to determine what 'smallest' actually means". — Nathan Pierson
– Nathan Pierson, Commented Sep 23, 2021 at 14:57

marolafm · Accepted Answer · 2021-09-23 15:12:50Z

1

Your question has nothing to do with lambdas, but with the std::sort function.

Indeed, if you read the documentation about the third parameter (the comparison function, the lambda in your case), it says:

comparison function object which returns true if the first argument is less than (i.e. is ordered before) the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(const Type1 &a, const Type2 &b);

Indeed, there is not need to create a lambda to pass it as the third parameter. You can pass any function object that receives two arguments of type T (the one of your container's elements) and returns bool.

For example, you can do something like this:

#include <vector>
#include <algorithm>
#include <iostream>

struct  {
    bool operator () (int a, int b){
        return a > b;
    }
}my_comp;



int main(){
    std::vector<int> v={1,2,3};

    std::sort(v.begin(), v.end(), my_comp);


    for(auto e:v) std::cout << e << " ";
    std::cout << std::endl;

}

answered Sep 23, 2021 at 15:12

marolafm

3552 silver badges9 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Marshall Clow Over a year ago

my_comp::operator() should be const

Collectives™ on Stack Overflow

Lambda function, arguments and logic in c++ [duplicate]

1 Answer 1

1 Comment

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Linked

Related